Monolayer Behavior of NbS2 in Natural van der Waals

Oct 23, 2018 - ... of China , Hefei , Anhui 230026 , People's Republic of China ... and Department of Physics, University of Science and Technology of...
0 downloads 0 Views 3MB Size
Download PDF
Letter pubs.acs.org/JPCL

Cite This: J. Phys. Chem. Lett. 2018, 9, 6421−6425

Monolayer Behavior of NbS2 in Natural van der Waals Heterostructures Wei Bai,†,§ Pengju Li,†,‡,§ Sailong Ju,⊥,§ Chong Xiao,*,† Haohao Shi,†,‡ Sheng Wang,⊥ Shengyong Qin,*,†,‡ Zhe Sun,*,⊥ and Yi Xie†

Downloaded via KAOHSIUNG MEDICAL UNIV on October 26, 2018 at 20:17:54 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ‡ ICQD, CAS Key Laboratory of Strongly-Coupled Quantum Matter Physics, and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic China ⊥ National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, People’s Republic of China S Supporting Information *

ABSTRACT: Monolayer transition metal dichalcogenides (TMDs) constitute an important family of materials with many intriguing properties and applications. The ability to achieve large-size and high-quality monolayer TMDs is the key prerequisite toward a deep understanding and practical application of TMDs in electronics and optoelectronics. Here, on the basis of high-resolution angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we find a monolayer NbS2-dominated Fermi-level feature in a misfit compound, which is a type of natural heterostructures. Considering the infrequency of the synthesis approach and electronic properties of the NbS2 monolayer, our results cannot only provide direct insight into the electronic structure of van der Waals heterostructures (VDWHs) but also shed light on the way toward rationally investigating the monolayer TMDs, which are hardly obtained and studied.

M

in 2H-NbS2. 3R-NbS2 is a nonsuperconducting metal without a CDW transition and shows an electron−electron interaction below ∼15 K.5 Therefore, it is natural to think of the electronic structure and transport property of single-layer NbS2, which is critical to comprehensively understand the superconductivity and the absence of CDW in 2H-NbS2 and the Coulomb interactions in 3R-NbS2. Meanwhile, ab initio calculations predict that the number of band dispersions crossing the Fermi level decreases from 3 to 1 as the thickness of NbS2 reduces from bulk to monolayer, which also implies that monolayer NbS2 may show unique physical and chemical properties different from its bulk counterparts.6 However, until now, the monolayer NbS2 could only be obtained by chemical vapor deposition with an extremely low yield,7,8 and its electronic structure and transport properties have rarely been reported. In the present work, we found monolayer behavior of NbS2 in a misfit compound (SnS)1.17(NbS2). We observed a monolayer NbS2-dominated Fermi level by angle-resolved photoemission spectroscopy (ARPES) and STM/scanning tunneling microscopy (STS), which endows us with a distinctive platform to pry the electronic structure of monolayer NbS2. Misfit compounds, bulk materials that

onolayer transition metal dichalcogenides (TMDs) have garnered plenty of attention and high expectations by academia and industry because their potential applications have opened new perspectives for engineering next-generation electronics and optoelectronics. However, their excellence would not be fully realized unless a large-area uniform and high-quality single-crystalline sample was available, which has stimulated many efforts to be made in the past years and is still a formidable challenge until now. NbS2, as a typical member of the TMD family, has attracted increasing interest because of its unique structure and remarkable electronic properties. At least two stable structure types of 2H and 3R phases can crystallize at room temperature. Both 2H- and 3R-NbS2 have the same monolayer NbS2 motif with the characteristic of sharing the lateral edges of trigonal prisms. However, 2H-NbS2 consists of stacking two monolayers with rotation, while 3R-NbS2 is formed by stacking three monolayers with in-plane translation. Differences in stacking mode result in different physical properties. 2H-NbS2 is a typeII superconductor with Tc ≈ 6.3 K and is not considered to show a charge density wave (CDW) state before entering the superconducting state as no special transition phenomena are observed by electronic transport1 and scanning tunneling microscopy (STM) measurements.2 However, very recently, phonon calculations3 and X-ray thermal diffuse scattering measurements4 gave traces of a latent CDW that continues the puzzle of the relationship between superconducting and CDWs © XXXX American Chemical Society

Received: September 10, 2018 Accepted: October 23, 2018 Published: October 23, 2018 6421

DOI: 10.1021/acs.jpclett.8b02781 J. Phys. Chem. Lett. 2018, 9, 6421−6425

Letter

The Journal of Physical Chemistry Letters

The electronic structure of (SnS)1.17(NbS2) was investigated by ARPES and STM/STS primarily. The constant energy contours of (SnS)1.17(NbS2) at different binding energies acquired by ARPES are illustrated in Figure 2a,b. The Fermi

consist of alternately stacked layers with a generic formula ([MX]1+δ)m(TX2)n (where M = Sn, Pb, Bi, Sb, or rare earth metals, T = group IV or V transition metals, X = S or Se, and δ is the degree of structural mismatch) belong to naturally occurring van der Waals heterostructures (VDWHs). Most of the previous literature focuses on the synthesis, crystal structure, and macroscopic properties of bulk misfit compounds, such as crystallography,9−13 optics,14,15 and electronic transport.16−18 A few articles also investigate their electronic structures by calculations19 and photoemission microscopy.20−22 Comprehending the electronic structures of misfit compounds, especially the specific feature of each sublayer, can provide important guidance for investigating the properties of monolayer TMDs. Misfit compound (SnS)1.17(NbS2) consists of one-by-one vertically stacked NbS2 and SnS sublayers. NbS2 and SnS sublayers can be redefined by an orthorhombic crystal system (space groups 38 and 39).23 Two sublayers share almost the same b-lattice constant, ∼5.75 A, but the ratio of the a-lattice constant for NbS2/SnS is ∼0.585 (=3.321/5.673). In one unit cell of (SnS)1.17(NbS2), the structural motif of SnS is 4 and that of NbS2 is 2; thus, the composition can be written as (SnS) 1.17 (NbS 2 ). (1.17 = 0.585 × (4/2)). Plate-like (SnS)1.17(NbS2) crystals were synthesized by chemical vapor transport (see the Supporting Information). The X-ray diffraction (XRD) pattern of Figure 1a indicates that the

Figure 2. Constant energy contours and band dispersions. Constant energy contours at a binding energy from (a,b) the Fermi surface EF to EB= −0.5 eV; (c) band dispersions of three high-symmetry directions marked in (a).

surface shows a pattern of near-six-fold symmetry. There is one big pocket around the Γ point and small pockets at the Brillouin zone corner K point. These pockets expand with an increase of binding energy, which means that the Fermi surface is dominated by two kinds of hole pockets. Below EF, these two dispersions of pockets merge into each other at EB ≈ −0.3 eV. Although lacking ARPES experimental literature for NbS2, calculated band dispersions of bulk crystal24 and monolayer6 NbS2 are available to compare with those of (SnS)1.17(NbS2). We find that the contours of the Fermi surface and band dispersions of (SnS)1.17(NbS2) from EF to EB= −0.3 eV are very similar to those of monolayer NbS2 instead of those of the bulk NbS2 crystal (Figures 2 and S2). Three bands cross EF in bulk NbS2, while only one band crosses EF in monolayer NbS2, which is in the same condition as (SnS)1.17(NbS2). A band gap exists from ∼−0.25 to −0.7 eV of binding energy in monolayer NbS2. However, in (SnS)1.17(NbS2), a new dispersion emerges at the Γ point below EB ≈ −0.5 eV (Figure 2b,c). We identify that this dispersion is mostly contributed to by the SnS sublayer. More evidence will be given in the following STS experiment. STM topographies and fast Fourier transform (FFT) images of SnS/NbS2 sublayers show honeycomb/orthogonal features (Figure 3). Note that the atomic structure of the NbS2 sublayer is clearly resolved by STM; however, the SnS sublayer is slightly blurred, but we can distinguish the orthorhombic pattern in the FFT pattern. Stair morphology crossing SnS and NbS2 sublayers can be found in STM topographic images (Figure 4a,d) on a cleaved (SnS)1.17(NbS2) surface. We used Pt/Ir tips to carry out the STS measurements on each sublayer (Figure 4b), which can directly probe the local density of states (DOS) of the sample surface.25 (We also tried tungsten tips for comparison and found no obvious differences.) Meanwhile, the aforementioned ARPES experiment can provide the energy

Figure 1. Crystal structure of (SnS)1.17(NbS2). (a) XRD pattern of single-crystal (SnS)1.17(NbS2), (b,c) STEM-HAADF along the [010] direction, and (d) corresponding ball-and-stick models.

repetition period of the z direction is ∼11.7 Å. In order to precisely determine the crystal structure, scanning transmission electron microscopy in high-angle annular dark field mode (STEM-HAADF) is presented in Figure 1b,c. The corresponding elements are indicated by colored spheres in Figure 1d. In the NbS2 sublayer, Nb atoms exhibit trigonal prismatic coordination. S atoms in the NbS2 sublayer are clearly demonstrated due to the great resolution to light elements in HAADF mode. Because of the overlap in the [010] direction, S and Sn atoms in SnS sublayers are difficult to distinguish. However, the features of stacked NbS2 and SnS heterostructures are unambiguously characterized by Figure 1b and electron diffraction (ED) along the c direction (see the ED and more details of the crystal analysis in Supporting Information Figure S1). 6422

DOI: 10.1021/acs.jpclett.8b02781 J. Phys. Chem. Lett. 2018, 9, 6421−6425

Letter

The Journal of Physical Chemistry Letters

The electronic transport properties of (SnS)1.17(NbS2) are illustrated in Figure 5. To some extent, (SnS)1.17(NbS2)

Figure 3. STM morphology. Atomic schematic, STM topographies, FFT, and self-correlation images of each sublayer.

Figure 5. Superconductivity and WAL effect. (a) Warming-up resistivity−temperature curve (the cooling-down curve shows no difference; data not shown). Temperature dependence of superconducting behavior under different fields shown by the inset illustration. (b) WAL behavior at small magnetic field.

behaves more like 2H- than 3R-NbS2, where the abnormal minimum resistivity in 3R-NbS2 caused by electron−electron scattering8 is not found in the whole measured temperature range while (SnS)1.17(NbS2) shows a sign of onset of superconductivity when the temperature decreases to ∼2.8 K, which is lower than that for bulk 2H-NbS2. (Figure 5a). The diamagnetic susceptibility curve is illustrated in Figure S3. When applying a magnetic field perpendicular to the ab plane, the magnetoresistance curve shows a “dip” at small field, which is a feature of weak antilocalization (WAL) (Figure 5b). WAL is a quantum conductivity correction to classical transport behavior, which originates from spin−obit scattering in a disordered low-dimensional system. In weak localization theory, carriers are scattered by four kinds of mechanisms: elastic scattering, inelastic scattering, magnetic scattering, and spin−orbit scattering. At such a low temperature, when WAL appears, elastic scattering can be negligible (the relaxation time is much larger than that of other mechanisms), and magnetic scattering is not presented in (SnS)1.17(NbS2). Under a small magnetic field, spin−orbit scattering will contribute a positive additional resistivity Δρ ∼ H2 while inelastic scattering will contribute a negative Δρ ∼ −H 2 . Therefore, in (SnS)1.17(NbS2), the observed dip-like resistivity character at small magnetic field is due to a normal parabolic magnetoresistance attached by a positive resistivity, which results from spin−obit scattering of 2D carriers. This dip appears when the temperature decreases to ∼3.8 K and becomes more obvious at lower temperature. The hole mobility is ∼140 cm2/V s around liquid helium temperature, which is obtained by fitting the magnetoresistance data at high field according to the Kohler equation (Supporting Information Figure S4). The relatively low mobility makes WAL and resistance oscillation untraceable at high temperature or magnetic field. Due to the infrequent report about electronic properties of monolayer NbS2, we are not able to affirm the existence of WAL in monolayer NbS2. Further studies are expected. In summary, by ARPES and STM/STS experiments, we are able to give a comprehensive description of the electronic structure of the misfit compound (SnS)1.17(NbS2). An isolated band dispersion dominates the DOS from EF to EB ≈ −0.3 eV, which is also the feature of monolayer NbS2 predicted by calculated works. The contribution of SnS sublayers arises

Figure 4. Electronic structure of sublayers. (a) Stair morphology of NbS2 and SnS sublayers, (b) STSs of NbS2 and SnS sublayers, (c) EDC acquired by integrating the ARPES signal intensity of 1/12 of the Brillouin zone, and (d) cross-sectional profiles of the white line in (a).

distribution curve (EDC) by integrating the spectral weight of a representative block of the first Brillouin zone, which can represent the overall information on the Brillouin zone (Figure 4c). Note that samples are exfoliated the same way under vacuum in ARPES and STM experiments; therefore, the EDC contains information on both sublayers. Because of the matrixelement effects of the linear light source of ARPES, the relative strength of the peaks in the EDC may not be accurate; however, the binding energies of characteristic peaks in the EDC can be a good reference for STS data. The EDC peak at EB ≈ −0.2 eV can be regarded as the local density of NbS2 sublayers around EF. The uprising differential current bias ∼−0.5 V on the SnS sublayer can fit well with the abovementioned ARPES SnS band dispersion, which arises around the Γ point at EB= −0.5 eV in Figure 2b. We notice that the differential current of the NbS2 sublayer also arises at around −0.5 V, which may result from the superposition of the porbitals of two sublayers; thus, both sublayers contribute to the DOS below −0.5 eV. 6423

DOI: 10.1021/acs.jpclett.8b02781 J. Phys. Chem. Lett. 2018, 9, 6421−6425

Letter

The Journal of Physical Chemistry Letters away from the Fermi level when EB is higher than −0.5 eV. The unique structure of the natural misfit compound extends the crystal size of monolayer TMDs to macroscopic order and presents properties of monolayer TMDs to some extent. By elaborately adjusting the species of sublayers, a variety of VDWH compositions can be obtained. We expect that (SnS)1.17(NbS2) will serve as a platform to explore prototypes of VDWHs and systematically study properties of monolayer TMDs, which are not available yet.

■

(5) Niazi, A.; Rastogi, A. K. Low-temperature resistance minimum in non-superconducting 3R-Nb1+xS2 and 3R-GaxNbS2. J. Phys.: Condens. Matter 2001, 13, 6787−6796. (6) Miró, P.; Audiffred, M.; Heine, T. An atlas of two-dimensional materials. Chem. Soc. Rev. 2014, 43, 6537−6554. (7) Wang, X.; Lin, J.; Zhu, Y.; Luo, C.; Suenaga, K.; Cai, C.; Xie, L. Chemical vapor deposition of trigonal prismatic NbS2 monolayers and 3R-polytype few-layers. Nanoscale 2017, 9, 16607−16611. (8) Zhao, S.; Hotta, T.; Koretsune, T.; Watanabe, K.; Taniguchi, T.; Sugawara, K.; Takahashi, T.; Shinohara, H.; Kitaura, R. Twodimensional metallic NbS2: growth, optical identification and transport properties. 2D Mater. 2016, 3, 025027. (9) van Smaalen, S. A superspace group description of the misfit layer structure of (SnS)1.17 (NbS2). J. Phys.: Condens. Matter 1989, 1, 2791−2800. (10) Kuypers, S.; Van Landuyt, J.; Amelinckx, S. Incommensurate misfit layer compounds of the type “MTS3”(M= Sn, Pb, Bi, rare earth elements; T= Nb, Ta): A study by means of electron microscopy. J. Solid State Chem. 1990, 86, 212−232. (11) Wiegers, G. A.; Meerschaut, A.; et al. Misfit layer compounds (MS)nTS2 (M= Sn, Pb, Bi, rare earth elements; T= Nb, Ta; n= 1.08− 1.19), a new class of layer compounds. Solid State Commun. 1989, 70, 409−413. (12) Ohno, Y. Modulation structure in the STM images of the nonmetallic misfit-layer compound (PbS)1.12VS2. Surf. Sci. 2006, 600, 4829−4837. (13) Bengel, H.; Jobic, S.; Deudon, C.; Rouxel, J.; Seo, D. K.; Whangbo, M. H. Scanning tunneling and atomic force microscopy study of the misfit layer compounds (LaS)1.14(NbS2)n (n= 1, 2) and [(Pb,Sb)S)]1.14NbS2. Surf. Sci. 1998, 400, 266−276. (14) Sourisseau, C.; Cavagnat, R.; Tirado, J. L. A Raman study of the misfit layer compounds,(SnS)1.17NbS2 and (PbS)1.18TiS2. J. Raman Spectrosc. 1992, 23, 647−651. (15) Hangyo, M.; Nishio, T.; Nakashima, S. I.; Ohno, Y.; Terashima, T.; Kojima, N. Raman and Infrared Spectra of Misfit Layer Compounds MNbS3 (M= Sn, Pb, La, Ce). Jpn. J. Appl. Phys. 1993, 32, 581−583. (16) Wiegers, G. A.; Meetsma, A.; Haange, R. J.; De Boer, J. L. Structure and physical properties of (SnS)1.18NbS2,“SnNbS3”, a compound with misfit layer structure. Mater. Res. Bull. 1988, 23, 1551−1559. (17) Nader, A.; Lafond, A.; Briggs, A.; Meerschaut, A. Critical Magnetic Field Measurements of the Misfit Layer Compound (SnS)1.17(NbS2) down to 50mK. Phys. Scr. 1998, 57, 310−312. (18) Falmbigl, M.; Fiedler, A.; Atkins, R. E.; Fischer, S. F.; Johnson, D. C. Suppressing a charge density wave by changing dimensionality in the ferecrystalline compounds ([SnSe]1.15)1(VSe2)n with n= 1, 2, 3, 4. Nano Lett. 2015, 15, 943−948. (19) Fang, C. M.; Ettema, A. R. H. F.; Haas, C.; Wiegers, G. A.; van Leuken, H.; de Groot, R. A. Electronic structure of the misfit-layer compound (SnS)1.17NbS2 deduced from band-structure calculations and photoelectron spectra. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 52, 2336. (20) Nakayama, K.; Eto, K.; Tanaka, Y.; Sato, T.; Souma, S.; Takahashi, T.; Segawa, K.; Ando, Y. Manipulation of topological states and the bulk band gap using natural heterostructures of a topological insulator. Phys. Rev. Lett. 2012, 109, 236804. (21) Yao, Q.; Shen, D. W.; Wen, C. H. P.; Hua, C. Q.; Zhang, L. Q.; Wang, N. Z.; Niu, X. H.; Chen, Q. Y.; Dudin, P.; Lu, Y. H.; Zheng, Y.; Chen, X. H.; Wan, X. G.; Feng, D. L. Charge Transfer Effects in Naturally Occurring van der Waals Heterostructures (PbSe)1.16(TiSe2)m (m = 1,2). Phys. Rev. Lett. 2018, 120, 106401. (22) Kalläne, M. Towards angle and space resolved photoemission: bonding in layered misfit compounds and development of reflective photon sieves. Doctoral dissertation, Universitätsbibliothek Kiel, 2007. (23) Meetsma, A.; Wiegers, G. A.; Haange, R. J.; De Boer, J. L. The incommensurate misfit layer structure of (SnS)1.17NbS2,SnNbS3’. I. A study by means of X-ray diffraction. Acta Crystallogr., Sect. A: Found. Crystallogr. 1989, 45, 285−291.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b02781.

■

Sample synthesis and characterization; TEM-ED and lattice structure analysis; calculated band structure; diamagnetic susceptibility; and mobility fitted by the Kohler equation (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.X.). *E-mail: [email protected] (S.Q.). *E-mail: [email protected] (Z.S.). ORCID

Chong Xiao: 0000-0002-6134-6086 Yi Xie: 0000-0002-1416-5557 Author Contributions §

W.B., P.L., and S.J. contributed equally.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Key R&D Program of China (2017YFA0303500, 2017YFA0205004), the National Natural Science Foundation of China (21622107, U1832142, 11621063, 11474261, 11634011, and U1532136), the Key Research Program of Frontier Sciences (QYZDYSSW-SLH011), the Youth Innovation Promotion Association CAS (2016392), the Major Program of Development Foundation of Hefei Center for Physical Science and Technology (2017FXZY003), and the Fundamental Research Funds for the Central University (WK2340000075, WK3510000006, and WK3430000003). This research was also supported by Beamline 13U of the National Synchrotron Radiation Laboratory.

■

REFERENCES

(1) Naito, M.; Tanaka, S. Electrical Transport Properties in 2HNbS2,-NbSe2,-TaS2 and-TaSe2. J. Phys. Soc. Jpn. 1982, 51, 219−227. (2) Guillamón, I.; Suderow, H.; Vieira, S.; Cario, L.; Diener, P.; Rodiere, P. Superconducting density of states and vortex cores of 2HNbS2. Phys. Rev. Lett. 2008, 101, 166407. (3) Heil, C.; Poncé, S.; Lambert, H.; Schlipf, M.; Margine, E. R.; Giustino, F. Origin of Superconductivity and Latent Charge Density Wave in NbS2. Phys. Rev. Lett. 2017, 119, 087003. (4) Leroux, M.; Cario, L.; Bosak, A.; Rodière, P. Traces of charge density waves in NbS2. Phys. Rev. B: Condens. Matter Mater. Phys. 2018, 97, 195140. 6424

DOI: 10.1021/acs.jpclett.8b02781 J. Phys. Chem. Lett. 2018, 9, 6421−6425

Letter

The Journal of Physical Chemistry Letters (24) Ivanovskaya, V. V.; Zobelli, A.; Gloter, A.; Brun, N.; Serin, V.; Colliex, C. Ab initio study of bilateral doping within the MoS2-NbS2 system. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 134104. (25) Kano, S.; Tada, T.; Majima, Y. Nanoparticle characterization based on STM and STS. Chem. Soc. Rev. 2015, 44, 970−987.

6425

DOI: 10.1021/acs.jpclett.8b02781 J. Phys. Chem. Lett. 2018, 9, 6421−6425